Re: [tsvwg] Status of ECN encapsulation drafts (i.e., stuck)

John Leslie <john@widor3.jlc.net> Tue, 17 March 2020 06:19 UTC

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Date: Tue, 17 Mar 2020 02:10:25 -0400
From: John Leslie <john@widor3.jlc.net>
To: Joseph Touch <touch@strayalpha.com>
Cc: Sebastian Moeller <moeller0@gmx.de>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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References: <FF777393-47B2-4B53-AD41-5883B2D67CC5@gmail.com> <264398ad-59eb-7cfd-0276-35ae0f0120a5@bobbriscoe.net> <44EB050C-C35C-47A0-BC78-3EEDB683B507@gmail.com> <c802dddc-8a55-47ea-9976-06771d39c952@bobbriscoe.net> <B3A657D0-EA9D-45EC-8003-21D158F83E06@gmx.de> <ea8fd9d7-82bc-7da0-a08d-31a2d46abe36@bobbriscoe.net> <D0036DCD-424F-46B0-819B-D9E60828CB50@gmx.de> <8acc44d5-d003-2a92-460e-81f31a26cc9b@bobbriscoe.net> <25F001C3-083B-442E-8CB7-88EEF32E1974@gmx.de> <B5996C0B-C5B6-4F33-B5F5-CA1522BC8DFC@strayalpha.com>
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Subject: Re: [tsvwg] Status of ECN encapsulation drafts (i.e., stuck)
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On Mon, Mar 16, 2020 at 08:41:50AM -0700, Joseph Touch wrote:
>> On Mar 16, 2020, at 2:12 AM, Sebastian Moeller <moeller0@gmx.de> wrote:
>> 
>>> [BB] I should add that PLPMTUD and clamping the max packet size
>>> complement each other. 
>>> 
>>> PLPMTUD without clamping would kill latency.
>>> Clamping avoids triggering fragmentation, but smaller than max
>>> packet sizes are inefficient. 
>>> Adding PLPMTUD eventually bumps up the MTU for each path to its max,
>>> removing the inefficiency.
>> 
>> [SM] Well, realistically the ONLY viable solution is to design all
>> tunnels to allow the de facto real-world internet MTU of 1500 and
>> just make sure the tunnel itself uses jumbo-frame capable links
>> to make sure fragmentation is not required...

   Never say "never!" (isn't that what "ONLY" means?)

   Yes, jumbo-frame capable links really _are_ essential...

>> But honestly, in our current context I still wonder whether an operator
>> failing to implement a non-fragmenting tunnel will ever deploy an
>> ECN-enabled AQM...

   Again, never say "never".

   Implementing non-fragmenting tunnels really isn't that hard! It's
just that so few people act as if they beleieve it necessary. :^(

> That won???t work either. Any tunnel that has an MTU of any type
> (e.g., even the max MTU for IP of 64KB) and can be used recursively
> either directly (X over X) or indirectly (X over Y over ??? over X)
> MUST support fragmentation somewhere.

   Yes, even 64KB MTU can be exhausted!

   We need better tunneling algorithms...

> Intermediate recursion tunnels can implement fragmentation at any
> of the protocols. Directly recursive tunnels MUST implement
> fragmentation (because nobody else can in way that helps).

   I'm not sure this is the best way to say it.

   The word "fragmentation" carries an awful lot of baggage. :^(

   I will own up to having done some _evil_ things in a past life,
such as running slow links with MTU less than 1500 to simplify life.

   We need descriptions of problems that come from running tunnels
without a tunnel-path MTU greater than the incoming MTU.

   (Of course there are things you can do as an alternative; but
they tend to be harder.)

   Where we get in trouble is pretending the problem can be ignored.
Alas, _many_ pointy-haired-bosses believe they can define the problem
out of existence. :^( :^(

> Bob???s doc has the (IMO) correct approach - tunnels are a pair of
> devices that both do work.
> 
> Joe

--
John Leslie <john@jlc.net>